(3) Direct Thermal Conversion by Electrostatics - Insufficient data available for this recently-proposed thermal engine. (4) Thermophotovol tai cs - rejected on consideration of overall efficiency and problems of waste heat rejection. In conclusion, the photovoltaic options utilizing either single crystal silicon solar cells or gal 1ium-aluminum-arsenide have evolved as options with the overall lowest cost and mass potential. A concentration ratio of one (no concentration) is preferred for silicon systems, whereas gallium arsenide designs may be more advantageous with a concentration ratio of two. Based on the analysis conducted to date, the results indicate these options would be less complex and would have lower mass than the thermal cycle options. The major disadvantages of the thermal cycle systems are fluid system containment, wear in rotating equipment, construction complexity, and relatively high mass. It should be noted, however, that the cost advantage of photovoltaics is very sensitive to array blanket costs. To illustrate this point, figure A-8 shows a cost trade-off between silicon photovoltaics and the potassium-Rankine cycle. At the projected solar blanket cost, the silicon system has a 5 to 10 percent cost advantage. With respect to the thermal cycle options, the recent system definition studies have indicated a preference for the Rankine cycle over the Brayton cycle. The Rankine system would use either a potassium working fluid or a cesium-steam working fluid combined cycle. This preference is not strong because it is based on small mass advantage relative to Brayton systems. Therefore, these systems are regarded as competitive at this time; however, as previously stated, the photovoltaic systems are preferred over the thermal systems. 2. Power Distribution The purpose of the power distribution systems is to collect, regulate and control power from the power generation system (solar array sections) and transmit this power via power busses and rotary joint/slipring system to microwave generators on the transmitting antenna. This system would also include energy storage as necessary to meet power requirements during the eclipse periods.
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